The present invention relates to a method and apparatus for continuously manufacturing calibrated round or profiled glass tubes, especially calibrated round or profiled glass tubes having a pre-determined inner profile and/or a pre-determined outer profile.
Glass tubes of the aforesaid type, having lengths of up to several metres are required as starting material for manufacturing a wide range of glass elements, for example, for manufacturing bottles and containers, tubular covers for illuminants and for manufacturing illuminant tubes in lighting engineering. On the one hand, it is required that these glass tubes should be produced as cheaply as possible. On the other hand, in many applications, for example in lighting engineering, there is a demand for glass tubes having precisely finished inner profiles, for example, predetermined by the technical specifications in the respective application.
In the manufacture of glass tubes, a fundamental distinction is made between discontinuous and continuous manufacturing methods. Because of the fundamentally different process parameters in most cases, the principles applied in discontinuous manufacturing methods cannot or at least cannot easily be transferred to continuous manufacturing methods so that they cannot serve as a stimulus for the person skilled in the art to improve continuous manufacturing methods.
DE 497 649 discloses a method and apparatus for drawing glass tubes having round or non-round inner profiles using the so-called Danner method. In this method liquid glass is externally guided onto an inclined, rotating tubular body and drawn off as a tube at its lower end. In this method the inner profile of the drawn glass tube is defined by the outer profile of the tubular body at its lower end. In order to produce glass tubes have a non-round inner profile, the lower end of the tubular body can also have an outer contour which deviates from a circular shape. The inner profile of the glass tubes thus produced has comparatively high tolerances. The edge radii of glass tubes having non-round profiles are also comparatively large. Thus, this method frequently no longer meets the present requirements for the precision and tolerances of calibrated round or profiled glass tubes.
U.S. Pat. No. 2,009,793 discloses the so-called Vello method for continuously manufacturing glass tubes having a circular inner profile. A substantially cone-shaped shaping body is arranged underneath an outlet opening of a melt channel, which serves as a receptacle for the molten glass, concentrically to the outlet member of the melt channel. The emerging molten glass is drawn out by a drawing device through the annular gap between the shaping member and the outlet member so that a glass tube having a circular inner profile substantially predetermined by the outer contour of the shaping body is formed. Downstream from the shaping member the still deformable, tubular glass body is further stretched by means of a drawing device until this has finally cooled below the softening point. The inside diameter and the wall thickness of the glass tube is determined by the dimensions of the annular gap between the shaping member and the outlet member of the melt channel, by the temperature of the emerging molten glass, the temperature relationships downstream of the outlet opening and by the tensile force or drawing speed of the drawing device. It is comparatively difficult to find suitable ranges of parameters for manufacturing glass tubes having different profiles and requires a great deal of experience which at least makes it expensive to completely automate the method. Re-equipping the melting furnace to manufacture glass tubes having different profiles is comparatively expensive. Nevertheless, glass tubes manufactured by the Vello method have tolerances which increasingly are no longer suitable for the present applications.
DE-OS 2 217 725 discloses a method for the continuous manufacture of a quartz tube. A shaping device having a core which determines the inner profile of the glass tube is located at the lower end of a continuously charged melting furnace in the area of the outlet opening. Profiled glass tubes having non-circular inner profiles can also be manufactured by exchanging the core. Re-equipping the melting furnace first requires this to be completely emptied before the core can be exchanged. This is comparatively expensive. The tolerances which can be achieved using this method frequently no longer meet the present requirements. Since very high temperatures are required to melt quartz glass, the process parameters and basic principles disclosed in DE-OS 2 217 725 differ very considerably from the process parameters and basic principles sought in the sense of this application. The principles disclosed in DE-OS 2 217 725 thus cannot be transferred to the subject matter of this application.
DD 254 380 A1 discloses a method and apparatus for continuously manufacturing internally calibrated glass tubes. Molten glass is drawn off from a drawing dish against the force of gravity. In this case, a drawing bulb is formed, which is drawn off at a vertically projecting drawing mandrel in a continuously tapering fashion. The drawing mandrel is used as a shaping body to define the inner profile of the glass tube to be manufactured. A still-deformable tubular body is formed which is drawn through a system of nozzles in which the glass tube sliding over the drawing mandrel is calibrated thereon by applying an underpressure. This process is supported by blowing pre-heated air onto the outside of the glass tube sliding over the drawing mandrel. In order that a glass tube having a constant wall thickness can be produced, the drawing bulb and the drawing mandrel must be rotated synchronously, which is expensive. Furthermore, it is comparatively expensive to control the parameters of the device so that the still-deformable glass tube can be suitably deformed by the nozzle system. Pre-coolers, after-coolers and heating devices need to be used for this purpose. This apparatus is only suitable for manufacturing calibrated round glass tubes and conversion to other inside or outside diameters is comparatively expensive.
SU 715511 and WPI-Abstract thereof disclose a method and an apparatus for manufacturing a calibrated quartz glass tube. The molten glass is drawn via a shaping body by means of a gripper. On the bottom of the shaping body and downstream in drawing direction are disposed a plurality of rods. The molten glass is drawn via these rods, whereby the inner profile of the glass tube is defined. Thus, glass tubes with a rectangular-shaped cross section can be manufactured.
The rods are, however, rigidly connected with the shaping body. In particular, the distance to the shaping body can not be varied. Thus, the method is only suited for a relatively narrow range of working temperatures and drawing velocities.
SU 837943 A and WPI-Abstract thereof disclose another method and apparatus for manufacturing a calibrated quartz glass tube, wherein the molten glass is drawn via a longitudinal profile forming boding disposed downstream of the shaping body. Rollers press the glass tube against the profile shaping body. However, the position of the profile shaping body relative to the shaping body cannot be varied. The parameters of the drawing method therefore can not be varied sufficiently. The glass tube is forcibly formed by means of only two rollers that are disposed opposite to each other.